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I've been following robot fighting ever since the first original series of RW, and it was a major reason why I went into Engineering in the first place.

12 years later, and I've finally gotten round to my first bot "Go Ugly Early". I'd been thinking about a 2WD electric front hinged flipper for a while, but recently the design has morphed into more of a boxy ram bot, with the ability to overturn anything which gets on top of it.

In the image, the front wedge itself doesn't go all the way to the floor, but two hinged metal pieces will drag along the floor, giving some it a front edge.

The chassis is made from a series of 20mm stacked aluminium extrusions, held together by a series of vertical bolts (not shown). The armour is a triangular strip of Arris rail pine (made for fence posts). As long as I've secured it properly, this fairly soft wood, this should absorb the impacts, and can be changed after every fight. Looking at other build diaries, I haven't seen many with this combination of materials, perhaps there's a good reason for that.

The model doesn't yet feature a few things, such as the battery, feathertwo and a series of rubber dampers to absorb some more impacts; but I've got several kg left in the allowance.

My key parts:
TX: Turnigy 9x (So far I'm happy, cheap, lots of channels, and mixing should I need it)
Feathertwo dual
Two Argos drills (mounts made by Roboteernat)
750N, 50mm Linear Actuator
4" 'stealth' wheels
For now just the original Argos battery, but this will be swapped out at a later date.

So far I have been focusing on getting the frame and mechanism just right. For the flipping/self righting mechanism, I found a program called Mechanical Expressions to be particularly useful, building a 2D structure, and applying the load of another robot on top.

The video shows a cross section of the robot, cut through the LinAc. The video shows that I can transition the top panel from 45 to over 100 degrees, all with the weight of another robot on top; while staying inside the LA's force limit of 750N.

I have also created the basic circuitry, and placed it in a testing chassis made from an off-cut plastic window ledge out of a skip.

I've started ordering a lot of the hardware now, so exciting times. Any thoughts and suggestions welcome.

Don't worry, I haven't settled on the exact shape yet (hence not on the model).
I'm thinking about having a second set of hinges on the front rail, and each will have a metal wedge attached which drags on the floor. Think, a lazy version of Panic Attack's forks.

Pine... I've used Pine before as a chassis for The Honey Badger 2.0. I got flipped out of the arena in Stevenage 2017 and the top one cracked completely. Changed to using Oak stair bannisters and that held up better until very recently.

To get under opponents, I'm thinking a strip of 4mm hardox which drags along the floor, hinged on the front beam. My only concern here is that any impact on the hardox will be transferred to the fire door hinge holding it, so I'm looking at a way of adding a damper to absorb some energy.
Just behind that beam is a second set of hinges which allow the movement of the LinAc operated wedge panel. I did look at using a single set of hinges, but in this configuration, if one set is damaged the other can still function.

I've also added my Feathertwo and batteries to the model, and while I still need to add a few bits and bobs, such as 14AWG wire, a few dampers, and a fair number of bolts; the total weight is only 10.6kg; so plenty of scope to thicken the materials.

I tried to have a single battery, but it's just too long to fit, so I might need to go for two. While slightly heavier, it will improve weight distribution, and give some redundancy.
One question I do have, do I go for 2x 4C 1300mAh in parallel, or 2x 2C 2600mAh in wired in series?
At the start of a fight both arrangements would give the same result. But should one battery fail, the parallel arrangement means the machine can still function, just for not as long. While having this redundancy is nice, do I risk draining the working battery? And if so, would two series batteries be the way to go?

Finally, I was looking at a ball caster for each of the front corners, but wondered whether I could use a dome head bolt (or nut) to slide along the arena?

No, they are in series, which, as you rightly point out, is a bit more risky than parallel because if the connecting wires break, that is it. The drain on the batteries will be equal though so you need to ensure that both batteries are fully charged as well otherwise you end up with one battery dying first, taking the other with it.

I use 2x 2S 2100mAh 2S1P LiFePO4 batteries in The Honey Badger 3.0, making it 4S1P but can go to 6S quite safely. The upshot of a series connection is more voltage so it is a easy way to get to 6S without needing a fairly weighty battery but the capacity remains 2100mAh so they do need charging after one or two fights. I keep a spare matching set around so I can simply swap out for the other set and charge the first set in the meantime.